1. Field of the Invention
The present invention relates to an antenna and more particularly to an antenna capable of concentrating radiating directions of lines of magnetic field.
2. Description of the Related Art
Rapid technological development allows rechargeable batteries to be used in wide range of products ranging from compact portable electronic products to electric cars/motorcycles. Electric power used to drive those products surely reduces environmental pollution and the clean power can be easily available to users. However, batteries are usually charged by wired battery chargers and the wired battery chargers from different manufacturers oftentimes differ from one another and are usually not interoperable because of incompatible connectors thereof.
In this regard, latest technique employs a wireless charging means to charge a battery. With reference to FIG. 9, an electronic product or an electric car/motorcycle has a wireless receiver 71 mounted thereon and a wireless power supply 72. The wireless power supply 72 has a RF (Radio Frequency) power amplifier 721 and a transmitting antenna 722. The wireless power supply converts an AC power into a DC power. The DC power is converted into electromagnetic waves by the RF power amplifier 721 and is transmitted to the wireless receiver 71 through the transmitting antenna 722. The wireless receiver 71 has a receiving antenna 711 and a rectifier 712. The receiving antenna 711 receives the electromagnetic waves and the rectifier 712 converts the electromagnetic waves into a DC power to charge an embedded battery. Although the electromagnetic induction between the transmitting and receiving antennae 722, 711 attains the wireless charging objective and gets rid of the operational inconvenience arising from the incompatible issues of the wired battery chargers and their connectors. Besides, magnetic leakage easily occurs if the transmitting and receiving antennae 722, 711 are not aligned to each other so that the efficiency of wireless power transmission is therefore reduced.
With reference to FIG. 10, a magnetic field distribution diagram of a ring coil 73 is shown. When the ring coil 73 is energized by current (I), according to the Ampere's right hand rule, the transmitting ring coil 73 will create a magnetic field (B). The lines of the magnetic field radiate outwards from the center of the ring coil 73 and return back to the center of the ring coil 73. With reference to FIG. 11, a schematic diagram of the distribution of magnetic field of wireless power transmission is shown. The two ring coils 73 are oppositely mounted. A zone of active transmission is formed between the ring coils 73, and two zones of inactive transmission are respectively formed around the distal outer edges of the ring coils 73. As being uniformly distributed, certain lines of magnetic field of the ring coils also pass through the zones of inactive transmission. When large current flows through the ring coils 73, electromagnetic interference (EMI) may arise from strong magnetic field distribution generated in the zones of inactive transmission.
With reference to FIG. 12, a network analyzer 81 is connected to a transmitting loop antenna 85 and a receiving loop antenna 84 through two respective impedance matching devices 83, 82. The network analyzer 81 sends a power signal to the transmitting loop antenna 85 via a first port (Port1) of the network analyzer 81, and the power signal is received by the receiving loop antenna 84 and is outputted to a second port (Port2) of the network analyzer 81. The network analyzer 81 analyzes parameters (S11, S21) by the received signals from the first port and the second port to obtain efficiencies of wireless power transmission of the loop antennae 84, 85.
However, when wirelessly transmitting power, the transmitting and receiving antennae 722, 711 of the wireless power supply 72 and the wireless receiver 71 or the ring coils 73 must be aligned to each other to avoid magnetic leakage during electromagnetic induction. Otherwise, magnetic leakage will occur and the efficiency of wireless power transmission is lowered. Additionally, surrounding electronic products are prone to the drawback of EMI because of the strong magnetic field distribution generated in the zones of inactive transmission in the electromagnetic field.
With reference to FIG. 13, as disclosed in U.S. Pat. No. 7,227,504 entitled “Gate antenna device”, a powered coil 91 and four powerless coils 92 on an integrated circuit (IC) constitutes a gate antenna. The powered and powerless coils 91, 92 are rectangular and planar. The powerless coils 92 are perpendicularly connected with four edges of the powered coil 91 to constitute a box with one side open. When the powered coil 91 is energized to generate electric wave signals, the powerless coils 92 will generate induced current due to electromagnetic induction so as to concentrate the electric wave signals generated by the powered coil 91 and to radiate the electric wave signals toward the open edge or the X axis. The powerless coils 92 formed around the powered coil 91 can concentrate the electric wave signals of the powered coil 91 instead of radiating the electric wave signals to all directions. However, as the powerless coils 92 generates power from the electromagnetic induction of the powered coil 91, part of the electrical wave signals of the powered coil 91 are consumed by the electromagnetic induction and the intensity of the electric wave signals of the powered coil 91 is therefore reduced.
From the above-mentioned descriptions, the transmitting antenna 722 of the wireless power supply 72 is prone to magnetic leakage and the wireless charging efficiency is lowered. Moreover, the issue of EMI arises from a strong magnetic field distribution generated in the zones of inactive transmission of the electromagnetic field. Despite the concentrating capability of electric wave signals, the powerless coils 92 of the gate antenna concentrate electric wave signals from the electromagnetic induction of the powered coil 91, certain electric wave signals of the powered coil 91 are consumed and the intensity of entire electric wave signals is also lowered.